Ozone has been used in reef aquaria for many years. It is claimed to have many benefits, ranging from increased water clarity to decreased algae. It has never, however, risen in popularity to the point where a seeming majority of reef aquarists use it. Many reasons likely prevent its widespread use, including its cost, complexity and safety concerns for both the aquarist and the aquarium's inhabitants. Speaking only for myself, my reasons for never having used it in my first ten years of maintaining reef aquaria were driven primarily by concern over ozone byproducts' toxicity in the aquarium, and the lack of a perceived need.


Back in the early to mid 1990s there was a fair amount of emphasis on ozone and other oxidizers as a way to raise the water's (the oxidation reduction potential). The ORP, in turn, was incorrectly described as a good way to measure the water's "cleanliness." So aquarists raised ORP. Then ozone and other oxidizers (such as permanganate) fell out of favor for a variety of reasons, not the least of which was the overall trend toward less technological approaches to reef maintenance.


It appears, however, that the use of ozone may be on the upswing. In a recent (December 2005) survey I did of experienced reef aquarists, the results were equally split between those who had never tried it, and those who were presently using it or who had in the past and would do so again in an appropriate aquarium. For most people who had used it, the emphasis is now on water clarity, not ORP as some surrogate of something that was vaguely defined but that was supposed to be beneficial.


This article is the first in a series that addresses the myriad issues around the use of ozone in reef aquaria. The articles should help aquarists understand why ozone is used and what molecular level processes take place when using ozone. Together, they should help aquarists determine for themselves if ozone is something they want to use, and if so, how to do so.

After a brief introduction to how ozone is used and some of its claimed benefits, this first article proceeds to describe what ozone is and how it reacts with seawater. It also relates ozone's perceived benefits to the actual chemical and biochemical changes that it can cause. In a sense, it provides the mechanistic framework for understanding why ozone does what it does, helps aquarists understand its limitations and details the changes in the aquarium water that ozone will cause, whether they are apparent to most aquarists or not (and, in fact, many are not).

The subsequent articles in this series will address the types of equipment necessary to effectively and safely use ozone, and the benefits that accrue upon initiating ozone in an aquarium system (mine) that had been operating for many years without it.

What is Ozone Supposed to Accomplish in a Reef Aquarium?

I've asked many aquarists what they believed dosing ozone accomplished in their aquaria. The list is always headed by increasing water clarity, but also includes other possibilities. Below, in no particular order, are the sorts of claims that are made:

1. Increased water clarity (even if it had been very clear before ozone)
2. Increased light penetration
3. Decreased yellowness
4. Decreased algae
5. Decreased cyanobacteria
6. Decreased skimmate production
7. Increased skimmate production
8. Increased ORP
9. Reduced nitrate
10. Decreased pathogenic bacteria
11. Reduced circulating toxins
12. Cleaner (more pure) water

Some of these make perfect sense, and the chemical and biochemical mechanisms that cause them through ozone's use will be detailed in the subsequent sections of this article. Others may not be correct assertions (decreased pathogenic bacteria, for example) and these issues are also discussed.

Some instances of apparent problems and perhaps underlying issues with the use of ozone are subtle enough that most aquarists never notice them. Bleached corals, for example, are obvious and have been reported. Perhaps the bleaching that has been experienced is related to a rapid increase in light penetration. But suppose that some small invertebrates in the aquarium were less prone to successfully reproduce due to residual bromate in the water. Or that the incidence of fish cancers from bromate (a suspected carcinogen) increased from, say, 1% to 2% for some particular fish species. How many reef aquarists would notice those changes, or attribute it to the ozone, even if it were true?

On the other hand, many aquarists might not particularly care about such subtle issues, and want the water to be clearer regardless of hypothesized problems. In any case, the data such as they are will be presented and aquarists can decide for themselves if ozone use is a practice they want to pursue or not. At the end of the last article in the series, where I present the results in my aquarium, I'll comment on whether I think it is desirable to continue using it or not in my system.

How is Ozone Used in Reef Aquaria?

How ozone is used will be the primary topic of the second article in this series, but in order to understand many of the issues presented in this article, it is necessary to have a rudimentary understanding of how ozone is used.

The pathway for ozone entering an aquarium starts with an ordinary aquarium air pump. The air travels out of the pump and often into an air dryer. The air's moisture is removed as it is absorbed by very hygroscopic solids. Not all aquarists perform this step, but removing the air's moisture has at least two benefits as the air passes into the next stage of the process. The next stage is a small device that generates ozone. The method used by most ozone generators is to pass the air through a high voltage electric discharge that breaks apart some of the oxygen (O2) molecules, and when they recombine, some ozone (O3) is formed (a second, less effective method uses UV light to accomplish the same process, either by passing air or the water itself past a UV light source). Moisture in the air reduces the amount of ozone formed in the generator, and it also results in the formation of nitric acid (HNO3; from water and nitrogen gas in the air). This nitric acid can reduce pH and alkalinity, and provides nitrate to the aquarium (which will be discussed in further detail next month).

After the ozone-containing air passes out of the ozone generator, it usually is sent into some sort of mixing chamber where aquarium water and the gas are mixed well, and are kept in contact for at least a few seconds. Aquarists often use skimmers or specially made ozone reactors for this purpose, and selection of suitable materials is a concern as the ozone can degrade some types of plastic, rubber and tubing. The amount of ozone delivered varies widely. Many manufacturers recommend on the order of 0.3 to 0.5 mg/hour per gallon of aquarium water, but many aquarists use less, or do not use it all of the time. They believe that using less ozone achieves their need for clearer water, reduces the need for more expensive equipment and air dryers, reduces concerns about toxicity due to byproducts and reduces its negative impact on skimming.

Inside the contact chamber, the ozone reacts with many different chemicals in the seawater including organics, ammonia, iron and other metals, bromide and iodide. It may also interact with viruses, bacteria and other organisms drawn into the chamber. The ozone itself survives for only a few seconds in seawater, but it leaves other reactive oxidizers (called ozone produced oxidants, OPO; for example, hypobromous acid, BrOH) in its wake. These can further react with organics and other materials and are also potentially toxic, so they should be removed before the water is released to the aquarium. Much of ozone's benefits happen in this chamber, where, for example, the water is made "clearer" as certain pigments in dissolved and particulate organic molecules are destroyed.

Water leaving the reactor is optimally passed over an amount of activated carbon sufficient to remove the remaining ozone produced oxidants. The carbon catalytically (and also noncatalytically) breaks down these oxidants before they enter the aquarium. The air passing out of the reactor also contains ozone, and is also best passed over activated carbon to reduce the aquarist's concern for airborne ozone's toxicity.

In order to ensure that not too much ozone enters the aquarium, aquarists should monitor ORP (the oxidation reduction potential) in the aquarium's water. For those aquarists using a small amount of ozone, monitoring may be adequate. For those aquarists using large amounts of ozone, an ORP controller is important. It can be used to shut off the ozone if the ORP rises above a set point (that point being either an emergency shut-off point that is rarely, if ever achieved, or a target ORP where the generator is actually running only part of the time, and only when the ORP controller says that ORP needs to be raised to the set point).

For comparison to other studies reported in this article, reef aquarists typically use up to about 0.3 ppm ozone in the "contact chamber" and have contact times on the order of a few seconds before the water passes into the aquarium. This value of 0.3 ppm ozone is based on adding ozone at a rate of 100 mg/hour (a typical addition rate suggested by ozone generator manufacturers for a tank of about 200 gallons) to a contact chamber (like a skimmer) that has a flow of 333 L/h; 100 mg/h / 333 L/h = 0.3 mg/L). Higher flow rates, lower ozone addition rates or incomplete transfer of the ozone into the water will give lower ozone concentrations in the contact chamber or skimmer.

Conclusion

Ozone has many effects when used in a reef aquarium. The most useful of these is the degradation of organic materials. Most importantly, and quite coincidently and fortunately for aquarists, the colored organic pigments in marine aquaria are very sensitive to ozone. For this reason, ozone can remove seawater's color quite readily, and much more effectively than it removes the overall load of organic material. Its effects on water clarity described by most aquarists range from minimal to very dramatic, with most aquarists reporting significant beneficial effects.

Another big effect of ozone is the bioavailability of the organics in the water. Many organics in the aquarium are not readily metabolized by bacteria, and such materials may last for hundreds or in the ocean. Ozone, however, has the ability to make many organic materials more readily absorbed and metabolized by bacteria. So in a sense, ozone triggers a bacterial attack that can reduce the load of circulating organic materials. This reduction in organic materials may also usefully apply to circulating toxins released by the aquarium inhabitants in an effort to kill each other with chemicals.

Ozone and its byproducts can, in high enough doses, kill many pathogens. The levels of ozone encountered in reef aquaria, however, may be inadequate to have any significant effect on total bacterial populations. Viruses are more susceptible than bacteria to ozone, and they may be effectively inactivated by typical use. Larger pathogens and parasites are much harder to kill and are not likely to be killed by ozone in reef aquaria.

Ozone also has a dark side. When reacted with seawater, ozone produces a variety of highly oxidized halogens such as BrOH and BrO3-. If the ozone produced oxidants are not largely removed with activated carbon, they may enter the aquarium and be hazards to the most sensitive organisms in the aquarium (which are likely eggs or early stage larvae).

Finally, ozone alters a variety of other inorganic materials in ways that may or may not be important. It alters the aquarium's redox balance, raising the ORP (which may mean as little as altering the ratios of different forms of manganese in solution). It may permit more rapid conversion of ferrous ion to ferric ion, and may increase its bioavailability, but perhaps decrease the lifetime of strongly complexed iron such as EDTA iron. Ozone also oxidizes ammonia to nitrate. While that is likely beneficial, it may alter the relative effectiveness of different nitrogen export pathways (macroalgae vs. denitrification, for example). It may drive the speciation of iodine toward iodate and away from iodide. Is that good or bad? I expect neither, although others have different opinions, but it is a good poster child for the many things that happen in reef aquaria when using ozone that normally take place without any notice or recognition of them by the aquarist.

So with all things considered, is the use of ozone in a reef aquarium worthwhile? Many aquarists answer with a resounding, "Yes!" I'll leave that question unanswered until additional information is detailed in the next two articles discussing what equipment and methods are most useful for applying ozone to aquaria, and reporting on what impact it had in my aquarium.